I actually messaged Amazon about this last week, just to see what was up. Here's what they said:

Unfortunately, we don't have any more stock of The Middle Length Discourses of the Buddha: A Translation of the Majjhima Nikaya (Teachings of the Buddha) item right now, and we're not sure if/when we'll be able to get more. Our supply of some items is limited, and these products sell out quickly. I realize this is disappointing news.We normally obtain the The Middle Length Discourses of the Buddha: A Translation of the Majjhima Nikaya (Teachings of the Buddha) from a supplier who can usually send it to us as soon as possible from the time your order is placed; however; in this case, the supplier has taken longer than expected to make this item available.

Our reliance on suppliers and manufacturers for information about their stock means that, occasionally, our database will not reflect all changes. I’m very sorry about this.

Rest assured that we're working with our suppliers so that we can try to have your order shipped at an earlier date. I hope you'll understand that we do our best to ensure that all of our customer orders leave our fulfillment centers as quickly as possible.

Typical big company stuff, but it appears that it has something to do with the supplier, not Amazon. I haven't contacted Wisdom Publications yet, but you could do so here if you'd like:

I contacted Wisdom, they said the Samyutta is "has been reprinted and is now available for purchase" and Majjhima " is with the printer now and will be available within two to three weeks". No mention of Digha.